The rivers that should nourish his thirsty rice paddies are too salty, and the rains are late this year. Dang Roi does not know if he will be able to salvage anything from this spring's crop.

Vietnam is the world's second-biggest rice exporter and the Mekong Delta, where Roi farms, accounts for more than half of its production.

But Roi's paddy fields in Ben Tre province are burning up during a drought which meteorologists say is the worst in decades.

The dry season should have ended already, but in the yard of Roi's house in Que Dien commune, barrels that collect rainwater for his family's cooking and washing show the desperate situation. They are half-full, or empty.

Experts say Vietnam is one of the countries most threatened by climate change, whose effects are seen in worsening drought, floods, typhoons, exaggerated tides, and rising sea levels.

The country is planning for a one-meter (three feet) rise in sea levels by 2100, which would flood about 31,000 square kilometers (12,400 square miles) of land an area about the size of Belgium unless systems such as dykes are strengthened, said a UN discussion paper released last year.

It said the threat of floods is greatest in the Mekong Delta, where 17 million people live.

If that land becomes unusable there are "serious implications" for the region, Helen Clark, administrator of the United Nations Development Program (UNDP), told AFP last month.

She said Vietnam faces a "huge challenge" from climate change.

Over the past 50 years the sea level has already risen by 20 centimeters (eight inches) along Vietnam's coast, according to the increasingly worried communist government.

While delta farmers cope with drought, they are also challenged by sea water intrusion, which experts also link to climate change.

There is little water in the rivers near Roi's fields "and it's salty so we can't pump it" for irrigation, he says.

The Vietnamese government emphasizes the role of climate change in disrupting its agricultural environment, but experts do not rule out an effect from dams upstream in China. That impact could be worsened by the opening of more dams further south in Laos and Cambodia, they say.

"The Chinese dams have made the system fragile, but the impact of the downstream dams will be cumulative," said Marc Goichot, of the WWF.

Goichot said a delta is influenced by three forces which affect one another: subsidence, which causes the delta's bed to fall; coastal currents; and sediment brought down by rivers.

Dams retain sediment, reducing the amount that collects where the coastal current and waves are strongest downstream, meaning the salty water can more easily penetrate, he said.

The impact of sediment needs to be better understood, Goichot added, calling for a suspension of dam projects pending further research.

China has eight planned or existing dams on the Mekong River, but rejects activists' criticism that the hydropower dams contribute to low water levels downstream.

There are proposals for another twelve dams in the lower Mekong countries.

Vo Tong Xuan, a leading Vietnamese rice expert, said the flow of the Mekong River whose long journey ends at the delta is "extremely reduced" this year.

He is concerned about the impact of Chinese dams, but also blames Vietnam's increasingly intensive methods of rice growing.

As the delta's population has expanded, farmers have gone from planting one to two and sometimes three rice crops each year.

Xuan says that too many farmers plant three crops, draining crucial water from provinces such as Ben Tre during the dry season.

Ultimately, he says, the Delta may need new varieties of rice more adapted to a dry and salty environment.

Roi, 64, grows rice only twice a year and is not waiting for new strains.

Squatting beside his sorry-looking paddies, he points out about 30 baby palm trees he has planted along the edge of the rice field. They are better adapted to the delta's harsh environment.

"If one day we can't grow rice any more, we'll grow coconut palms," he says.